An optical scanning device, for example provided in a laser printer, utilizes an oscillating mirror (micrometer) other than a polygon mirror driven to rotate by a motor. This optical scanning device is configured such that the oscillating mirror is driven to rotate by an electrostatic force so that laser light from a light source is deflected by the oscillating mirror to thereby perform reciprocating scanning on a photosensitive material. FIG. 10 is a view showing the relation between the rotation angle of the oscillating mirror and the scanning locus of laser light in a related-art optical scanning device. Such laser light scans on the photosensitive material in its center portion (solid line portion in FIG. 10) except its both end portions (broken line portions in FIG. 10). As apparent from FIG. 10, the distances d1, d2 between ends of adjacent main scanning lines R in a sub-scanning direction varies in the configuration in which laser light reciprocatorily scans on the photosensitive material by use of the oscillating mirror. For this reason, a density difference may occur between the both ends in a main scanning direction because of this scan skewing.
Japanese Patent Application Publication No. 2007-62097 describes a technique in which a plurality of cells each composed of a plurality of pixels and having a dot pattern formed according to the gradation of an image are separated from each other by odd-number pixels in the sub-scanning direction to thereby suppress the density difference in the main scanning direction on the entire image.
However, in the technique, it is impossible to obtain an effect of suppressing the density difference unless a certain large number of cells are formed. In addition, there is a problem that blank regions corresponding to the odd-number pixels are formed between the adjacent cells.